1. Field of the Invention
The present invention is directed to a driver circuit for generating pulses of the type used in fast digital transmission systems and in measurement technology.
2. Description of the Prior Art
Pulses having relatively high power which are as symmetrical as possible with respect to a given threshold and to the edge steepness are often required for various types of applications, such as for fast digital transmission systems and in measuring equipment. Such pulses are generated by driver circuits. A driver circuit usually has an output stage containing at least one differential amplifier. During operation of the driver circuit in a differential mode, it is relatively easy to generate pulses which are symmetrical with respect to a given threshold and to the edge steepness. This type of operation, however, is too complicated for many uses.
There are also applications wherein operation in a differential mode is not possible. For example, a laser diode in a transmitter of an optical fiber transmission link cannot be driven with a driver circuit operating in a differential mode. Although the output stage of a laser driver is usually formed by a limiting differential amplifier, pulses for driving a laser diode can only be tapped at one of the two complementary outputs of the differential amplifier. This type of operation is referred to as single-ended operation. The shape of the pulses which are obtained using single-ended operation is often asymmetrical. The pulse shape is necessarily asymmetrical in circuits having bipolar transistors, because the output current pulse occurring when the transistor is turned on has a noticeable overshoot, and moreover the signal edge is more highly delayed at the time of turn-on of the transistor than when the transistor is turned off. Because the collector current cannot change its operational sign, no undershoot occurs in the output current toward the end of the time of turn-off. In the case of a laser driver, asymmetrical pulses lead to a reduction in the length of a transmission link which is possible without a repeater because of the time jitter which occurs.
It is known to improve the symmetry of a pulse generated by single-ended operation of a driver circuit by operating the driver stage of the circuit with smaller input voltage swings. As a result, however, the edge steepness of the pulses is diminished. This results in a possible reduction in the data transmission rate.
It is also known to improve the symmetry of such pulses by including an intentional pre-distortion of the generated pulses using passive components such as, for example, filters. This technique, however, requires a complicated matching of components and does not allow monolithic integration. The optimization of the passive components, moreover, is highly dependent on the transmitted data rate, and on the properties of the driver circuit and its load.
Another known possibility is the use of hybrid components (for example, a 180.degree. hybrid) for adding the two complementary signals at the outputs of the differential amplifier to form a symmetrical signal, given simultaneous inversion of one of these two signals. This technique, however, has the disadvantage of being extremely complicated and not particularly flexible and, moreover, does not allow monolithic integration. Additionally, this known technique results in a limitation of the bandwidth, which is frequently undesirable.